A terminal reheat system has a central air handler that delivers tempered air to all zones. Before the air is delivered to any zone, however, it passes through a terminal unit that heats the air up to match the load requirements of the space served. This arrangement permits very exact control of the environment, as it can deliver whatever volume of air is needed, at the proper temperature. This is accomplished at an energy loss, however, because the system consistently cools and then reheats the same air or water. This is an acceptable penalty if exact temperature and humidity control is really needed; otherwise, there are several things that can be done to make the system more energy-efficient, at the loss of some space temperature control:
1. Reduce the volume of air delivered to each of the terminal units, and thus the total volume delivered by the central air handler. This will reduce the fan horsepower as well as the heating and cooling that must be provided by the hydronic fluid.
2. If precise temperature and humidity control are not required at any of the zone units, convert the entire system to a variable air volume system by adding a variable-frequency drive to the fan motor and changing out the terminal units to VAV boxes. Only exterior zones will need reheat coils, if at all.
3. Even if very accurate temperature and humidity control are needed, the water temperature can be lowered, and the flow to the reheat coils can be reduced. This, in turn, reduces the available control but saves more energy than it costs in lost convenience and control.
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Excerpt from Energy Conservation Projects.